1. Field of the Invention
This invention relates generally to optical communication systems and particularly to optical laser sources with multiple lasing wavelengths. More particularly still, it provides multi-channel laser signals by utilizing the beat signal of two lasers followed by a non-linear fiber multiplier.
2. Prior Art of the Invention
Dense wavelength division multiplexing (DWDM) offers a very efficient method to exploit the available bandwidth in the low attenuation band of the optical fiber. In this technology, the enormous available bandwidth is chopped into a number of parallel wavelength channels, where each channel carries data up to a maximum rate compatible with electronic interfaces. Furthermore, different protocols and framing may be used on different channels. This is very similar to frequency division multiplexing (FDM) used for radio and TV transmissions. As technology progresses the number of feasible channels in the total band increases. Early WDM systems used only 4 to 16 channels, while new systems are targeting more than 100 channels.
The low attenuation wavelength band includes different wavelength sub-bands. The first band used in modem optical communications is called Conventional Band or C-Band. This band includes wavelength channels from 1520 to 1565 nm. As demand for more bandwidth increased, the number of channels in the C-Band could not provide the capacity required by modem telecommunication networks. Therefore, longer and shorter wavelength channels were introduced. Wavelengths covering 1565 to 1610 nm form the Long Band or L-Band, while 1475 to 1520 nm form the Short Band or S-Band.
In the transmitter side of a WDM system, there are a number of different laser sources with different wavelengths. Each data channel is modulated on one of the wavelength channels and all the wavelength channels are then multiplexed and transmitted via the same optical fiber. At the receiving end, each channel must be demultiplexed from the set of wavelength channels. An optical receiver, then, will demodulate data from each channel. The capacity of a WDM system increases as more wavelength channels are provided. It is therefore desirable to increase the number of channels, decrease channel spacing and increase the total wavelength window.
Present DWDM systems need a large number of laser sources as well as techniques to modulate data signals on each source, combine, demultiplex and detect each data stream. The present invention addresses the important requirement for laser sources. In particular, it provides a multi-wavelength laser source that simultaneously furnishes a number of wavelength channels.
Currently, laser sources used in DWDM systems are exclusively of the single-wavelength variety. Distributed Feed-Back (DFB) lasers, Fabry-Perot lasers and ring lasers are some of the main technologies. Each wavelength supported in the system has a dedicated laser and its ancillary electronics. In the last few years and still today, the majority of lasers used are capable of emitting light only at a fixed wavelength. Increasingly, however, designs are making use of tunable wavelength lasers, which have broader spectral range and can operate at any point within that range. The primary drawback of both of these devices, however, is the sheer number that is required to satisfy high channel count systems proposed for the future optical network. At the same time, it is important to be able to lock the center wavelength of each laser source to a specific wavelength. This is mainly due to the fact that if there is any drift in the wavelength of a laser, it can interfere with the adjacent wavelength channel. This fact imposes a practical limitation on the number of discrete laser sources that may be placed in a very tightly spaced wavelength channel system providing a large number of channels. As a result, a multi-wavelength laser source that can provide an efficient and simple wavelength locking system is highly desirable.
This invention provides a novel design which provides simultaneously a number of wavelength channels. The present design requires only a single wavelength locking mechanism to tune and lock the entire set of channels to the ITU standard grid. Furthermore, the present system is able to provide wavelength channels in all three S, C and L bands.
In the system of the present invention, two DFB laser outputs are combined in a first stage to produce a beat signal. The two main channels interfere with each other to form beat signals. This combined signal is then used as the seed to create multi-channels through optical fiber non-linearity in a multiplier stage. The multiplier stage expands the two initial channels to cover the target wavelength band. A Comblike Dispersion Profile Fiber (CDPF) system is used in the multiplier section. The channel spacing of the resulting channel set follows the channel spacing of the two initial DFB lasers.
Accordingly, a multi-wavelength laser source (MWLS) system, comprising first and second monochromatic lasers having first (f1) and second (f2) lasing frequencies, respectively, means for amplifying combined signals of said first and second lasers and means for multiplexing the amplified combined signals to yield Comblike multi-channel laser signals separated from each other by a frequency equal to the difference between f1 and f2.
The system as defined above, said means for multiplying comprising a plurality of serially interconnected optical fiber sections each having predetermined propagation characteristics for said amplified combined signals, said predetermined propagation characteristics being propagation mode, dispersion and length.